In the past, electronic parts were most commonly axial parts having long leads extending from one of the ends of the parts. As electronic parts became smaller and came to require a larger number of leads, electronic parts were developed having short leads disposed on the periphery of the parts. Examples of such parts with peripheral leads are single in-line package (SIP) having leads on one side of a package, and dual in-line packages (DIP) having leads disposed on two sides of a package and quad flat packs (QFP) having leads disposed on four sides of a package. While parts with peripheral leads represent a great improvement with respect to mountability compared to axial parts, they still have a limit with respect to the number of leads which can be installed on a part.
More recently, electronic parts having electrodes provided on the bottom surface of the bodies of the parts have become popular. The bottom surface of the body of an electronic part has a greater surface area than do the sides of the body, so the number of electrodes which can be provided can be greatly increased compared to the number of leads on a part with peripheral leads. To the extent that the number of electrodes of an electronic part can be increased, the number of functions capable of being performed by the electronic part can be increased. A typical example of an electronic part having electrodes on its bottom surface is a BGA (ball grid array) package.
A BGA package typically includes a substrate having a semiconductor integrated circuit (IC) chip mounted on its top surface and an array of electrodes formed on its bottom surface. A rounded mass of solder, referred to as a solder bump, is attached to each of the electrodes. The BGA package can be connected to a printed circuit board, for example, by placing the BGA package atop a printed circuit board with each of the solder bumps of the package contacting a corresponding electrically conducting land of the printed circuit board, and then heating the BGA package and the printed circuit board so as to melt the solder bumps and solder them to the lands. Each of the solder bumps forms a minute soldered joint which mechanically and electrically connects the BGA package to the printed circuit board. The use of solder bumps is advantageous in that it enables a large number of uniform soldered joints to be simultaneously formed on all of the electrodes of a BGA package.
BGA packages can have a wide range of sizes and structures. When a BGA package has roughly the same planar dimensions as the integrated circuit chip mounted on its substrate, it is classified as a CSP (chip scale package). When a BGA package includes a plurality of IC chips, it is classified as a MCM (multi-chip module).
The solder bumps of a BGA package can be formed by a number of methods. One commonly used method employs solder balls. Solder balls have a uniform weight, and their spherical shape makes them easy to supply to appropriate locations on a substrate, so they have an optimal shape for forming bumps on a BGA.
In the past, solder balls for use in forming solder bumps on BGA were most commonly alloys of Sn and Pb, and particularly a 63Sn—Pb alloy, which is the eutectic composition of Sn—Pb alloys. The eutectic composition has a low melting temperature of only 183° C., which is a suitable temperature for avoiding thermal damage of electronic parts, and it also has excellent wettability with respect to the electrodes of a BGA package and the lands of a printed circuit boards. Therefore, a 63Sn—Pb has the excellent property that it produces few soldering defects.
However, it has been found that the use of lead-containing solders, including lead-containing solder balls, is a source of environmental pollution. When electronic parts soldered with a Sn—Pb solder malfunction or become old and are no longer convenient to use, they are disposed of by being discarded. When such equipment is discarded, some portions of the equipment are capable of being reused or recycled. For example, plastics in cases, metals in frames, and precious metals in electronic parts are often recovered. In contrast, a printed circuit board with solder being bonded typically cannot be reused or recycled. Therefore, discarded printed circuit boards are usually pulverized and then disposed of by burial in landfills.
If a printed circuit board which is disposed of by burial employs a lead-containing solder, such as a Sn—Pb solder, and if the printed circuit board is contacted by acid rain having a high pH, lead in the Sn—Pb solder can be dissolved out and mixed with rain water and enter underground water supplies. If humans or livestock drink underground water containing lead over a long period, the lead can accumulate in the body and may cause lead poisoning.
To avoid the environmental and health problems associated with the use of lead-containing solders, there is a movement in the electronics industry towards the use of so-called lead-free solders which do not contain lead.
The most common types of lead-free solders are Sn—Ag based solders, Sn—Cu based solders, Sn—Bi based solders, and Sn—Zn based solders having Sn as a principal component, to which one or more of Ag, Cu, Zn, In, Ni, Cr, Fe, Co, Ge, P, Ga, and the like may be suitably added as additional alloying elements.
Thus, there are various types of lead-free solders. Each types has its own strengths and weaknesses, so the various types differ with respect to use.
In Japanese Patent No. 3027441, the present applicant disclosed a lead-free solder in which Cu is added to a Sn—Ag alloy. Of the compositions disclosed in that patent, Sn-3Ag-0.5Cu is superior with respect to properties such as solderability, bonding strength, and resistance to thermal fatigue. Therefore, at present, that alloy is much used in the soldering of electronic devices. It is also used to form solder balls for forming bumps for BGA packages.
When a Sn—Ag—Cu based lead-free solder is formed into solder balls, the surface of the solder balls may not be perfectly smooth and may have surface irregularities. Such surface irregularities are undesirable because they may prevent the solder balls from smoothly rolling and may prevent the solder balls from being accurately supplied to a mounting apparatus for mounting solder balls on a BGA substrate. In order to eliminate irregularities on the surface of a Sn—Ag—Cu based lead-free solder, Japanese Published Unexamined Patent Application No. 2002-57177 discloses solder balls in which 0.006-0.1 mass % of at least one of Ge, Ni, P, Mn, Au, Pd, Pt, S, Bi, Sb, and In is added to a Sn—Ag—Cu based alloy.
The solder balls of that patent application do not have surface irregularities, so they can be smoothly and accurately supplied to a mounting apparatus for solder balls. In addition, they have a high bonding strength. According to that patent application, Ge has the effect of preventing oxidation, Ni, P, Mn, Au, Pd, Pt, S, and In have the effects of lowering the melting point and increasing the bonding strength, and Sb has the effect of increasing strength.